Effect of Alkaline Pretreatment on Characteristics and Biogas Production of Peanut (Arachis hypogaea) Shells

Abstract

Anaerobic digestion (AD) mitigates high energy demand by converting lignocellulosic waste into biogas and digestate. This process supports the economy, reduces energy crises, and enhances environmental sustainability. Agricultural residues serve as promising energy resource in carrying out AD process. Recalcitrant lignocellulosic structure of Arachis hypogea shells and high carbon to nitrogen (C/N) ratio of peanut shells (PS) usually impedes efficiency of AD process. Therefore, this study was conducted with two aims i.e. application of alkaline pretreatment on PS before initiating AD and use of vegetable waste (VW) as a co-substrate to balance the C/N for effective AD performance. Exposure of PS was given to varying NaOH

concentrations of 2%, 4%, 6% and 8%, which was followed by batch-mode AD including mono- and co-digestion under mesophilic temperature range for 45d. The characterization of untreated

and pretreated PS was done using FT-IR (Fourier Transform Infrared) spectroscopy. FT-IR results confirmed the change in structural peaks after pretreatment. Additional ultimate analysis depicted the enrichment of essential nutrients in VW, making it suitable for using as co-substrate with PS. Results showed that 4% NaOH provided maximum cellulose recovery (96.04%) and 75% lignin removal. Further investigation revealed an improvement in biogas production during co-digestion setup rather than mono-digestion. Anaerobic co-digestion (ACD) of NaOH pretreated PS with VW has recorded cumulative biogas production in following order: 4% > 2% > 6% > 8% which produced 65% , 58.75%, 42.2% and 37.5% respectively more cumulative biogas as compared with control group. Overall, low NaOH dosage i.e. 2% to 4% was found more effective than high dosage 6% to 8% in delignification. This study concluded that alkaline pretreatment has effectively improved cellulose recovery and lignin degradation, while ACD resulted in greater biogas production than mono-digestion.